A polysaccharide is a long chain of monosaccharide molecules, held by glycosidic bonds. They are usually not sweet in taste, insoluble in water and often do not produce crystals when water is taken out.
Starch is a complex solid carbohydrate, consisting glucose molecules held together by glycosidic bonds. It is a storage polysaccharide. They can be found in fruits, seeds, roots and other parts of the plant. The monomer of starch is glucose. Therefore, starch molecules can be made by polymerisation reaction, where glucose molecules are joined together to form a long chain.
These starch molecules are held by glycosidic bonds. Uses of starch
Starch is a polysaccharide, so it has very large molecules. This means they are insoluble, so they are suitable for storage because they do not do osmosis, do not easily diffuse out of cells; is compact as a result of its glycosidic bonds’ angles giving it a coiled structure, making it possible for them to be stored in small places.
It is also made up of small sub-units of alpha-glucose, making it easier for enzymes (amylase) to break down the molecule for an efficient release of glucose for respiration.
Glycogen is a highly branched polysaccharide which is the main form of storage in animals. It has a similar structure to starch, but glycogen has shorter chains.
It is mostly stored in small granules in animals Glycogen structure is similar to the structure of starch. Therefore, it is a large molecule, making it insoluble so it is suitable for storage in animals. It is also compact because of its glycosidic bonds giving it its coiled structure, so they can be stored in small places. However, they are made up of smaller chains so they can be easily hydrolysed into alpha-glucose for more efficient respiration.
A molecule of glycogen is made up of hundreds of units of glucose, branching off every ten glucose molecules or more. These molecules are joined together by a condensation reaction, held by glycosidic bonds.
Cellulose is a polysaccharide which is the main part of every plant tissue, consisting of long unbranched chains of glucose units which are linked. Cellulose differs from starch and glycogen as it has a straight, unbranched chain instead of a coiled chain. Apart from this, the major difference between cellulose and starch and glycogen is that instead of alpha-glucose monomers, there are beta-glucose monomers. This however causes major differences in the structure and function of the cellulose. This is mainly because of the reverse positions of the –H and –OH groups.
The –OH groups, instead of being below are above the ring. Therefore, every beta-glucose molecule must be rotated 180 degrees to molecule next to it, in order to form glycosidic bonds. This means that there is an alternation with –CH2OH group on every beta-glucose molecule, from being either above or below the chain. The cellulose molecules are grouped together, forming microfibrils, making them parallel to each other, so there are hydrogen bonds in between adjacent chains. The large numbers of hydrogen bonds still strengthen the cellulose, hence why it is good for structural purposes. Cellulose is important as they give the rigidity and strength of plant cells.
The bonds between the molecules are strong, making them hard to break down/digest. Also, cellulose cell walls prevent water entering by osmosis so they do not burst as they apply inward pressure to prevent any more water entering. Therefore, the plant cells become turgid. This helps maintain stems and leaves to be turgid so there is a lot of surface area for photosynthesis.
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